Field of the Invention
The invention relates to a method and a control unit for carrying out a gas exchange in a cylinder of an internal combustion engine. The invention also relates to an internal combustion engine having such a control unit, and to a motor vehicle having such an internal combustion engine.
In internal combustion engines, a gas exchange takes place after every combustion process. The manner in which this gas exchange is carried out substantially influences the fuel consumption and the pollutant emissions of the internal combustion engine. It is necessary, therefore, to configure the gas exchange in such a way that a cylinder charge contains certain portions of fresh air, residual gas, and purge air.
In a four-stroke engine, the four process steps of intake, compression, combustion, and exhaust are usually carried out. In a conventional Otto-cycle engine, the intake valve is opened during intake, shortly before the piston has reached top dead center, wherein the exhaust valve remains open. After top dead center has been passed through, the exhaust valve is closed. The intake valve is closed again only after the piston has reached bottom dead center. An internal combustion engine based on the Miller principle differs from an Otto-cycle engine in that the intake valve closes very early as compared to the Otto-cycle engine, whereby the quantity of air in the cylinder is reduced. Given that the intake valve has already been closed before bottom dead center is reached, the expansion volume is increased without the compression pressure being increased, whereby fuel is saved and the temperature in the cylinder is lower than in an Otto-cycle engine.
There are different ways to control the portions of fresh air, residual gas, and purge air in the cylinder charge. It is known, for example, to regulate the charging of the cylinder in a suitable way by adjusting a throttle valve in the intake manifold, whereby the intake manifold pressure is changed. Due to the large volume of the intake manifold, pressure changes set in with a delay, and therefore a charge control via the throttle valve is sluggish. Alternatively, there are different approaches for controlling the cylinder charge via the valve train. Such a charge control requires, however, that the pressure conditions in the intake manifold and in the manifold are precisely known. This is not readily possible with regard to the exhaust gas back pressure in the exhaust port or in the manifold, which has a non-negligible influence on the cylinder charge.
International Application WO 2014/16926 A1 describes a method for charging an engine. In this case, as a result of a torque demand, opening and closing times of an intake valve are determined on the basis of the intake manifold pressure, through the use of a characteristic map, and a target intake manifold pressure is determined on the basis of the opening and closing times of the intake valve. This target intake manifold pressure is then used as the basis for adjusting the throttle valve and the exhaust valve.